too darn hot

by Christina Hulbe

I remember July of 1995 pretty clearly. For nearly a week, my second-floor walk up was, like most other folks' apartments in my neighborhood, unbearably hot. It's not uncommon to be without air conditioning if you are poor or live in the older core of a major city. I felt physically ill, sat in a bathtub of cool water each night before trying to get to sleep, and thought about camping out in my air-conditioned office. Some city Aldermen (and women) were touring their wards, knocking on doors, looking for older residents in distress. By the end of the Chicago heat wave of 1995 , somewhere near 700 people had died of heat-related illness (up to 1000 throughout the region), many of them elderly and alone.

We're in for a stretch of hot (for us) days here in Portland and that's got me thinking about heat waves, a major public health risk that is projected to become worse with continued global warming. The Glossary of Meteorology defines a heat wave, broadly, as a "period of abnormally and uncomfortably hot and usually humid weather." The U.S. National Weather Service uses a heat index, which combines temperature and relative humidity, to issue heat advisories and warnings. What's most important is that extreme events (of any kind) transpire in a regional context. A string of 90-degree Fahrenheit (F) days in Phoenix does not have the same public health implications as string of 90-degree days in Chicago. The difference lies in such things as humidity, night-time cooling, landscape effects, and cultural adaptation.

meteorology
Heat waves are associated with semi-stationary warm anticyclones. Anticyclonic circulation in the atmosphere, clockwise in the northern hemisphere, moves air toward the center of the circulating cell (flash animation). The upper-level convergence increases atmospheric pressure. You can think of this as a pile-up of warm air in the center of the anticyclone that produces high pressure under all that extra mass. The high pressure produces divergent flow down at the surface and thus, downward vertical motion within the warm, high-pressure cell (cartoon). The warm air compresses as it sinks, a process that warms it further and inhibits cloud formation. (In contrast, rising warm air expands and cools, often producing clouds as vapor in the air condenses.) An excellent description with maps of normal and anomalous summer conditions is available at this National Weather Service website.

Meteorologists identify high and low pressure systems using maps of the height of constant-pressure (isobaric) surfaces within Earth's atmosphere. Pressure decreases with altitude (less and less atmospheric mass is up above you) so the 500 millibar (mb) surface is lower than the 100 mb surface. At sea-level, the average atmospheric pressure is about 1013 mb. The 500 mb surface, representing mid-troposphere conditions, often appears in such analyses. About half the mass of Earth's atmosphere lies above and half below this level. In forecasting, meteorologists and climatologists look at not just the values of particular diagnostic quantities but how they differ from mean conditions. The difference from mean conditions is called an anomaly. For example, here is a NOAA animation of 500 mb heights and anomalies. Unusually high pressure aloft, as we would find during a heat wave, is assoicated with increased height of the 500 mb surface, a "positive height anomaly" (cartoon).

future trends
Climate model studies of extreme heat events in a global-warming world project more intense, frequent, and lengthy heat waves as we progress through the 21st century. The trend is not simply a result of warm seasons growing warmer, but a distinctive pattern connected to atmospheric circulation.

Gerald Meehl and Claudia Tebaldi (Science, 305 (5686), 994 - 997; pdf of technical paper; BBC story), climatologists at the U.S. National Center for Atmospheric Research, studied the effect of global warming on heat waves by comparing modeled climates for a suite of greenhouse gas emissions scenarios with the present climate. They found that global warming amplifies positive height anomalies for the 500 mb surface (anomalies grow larger). Put another way, you get more bang for your greenhouse gas buck in regions already characterized by summertime warm high-pressure cells.

The models project changes is in both intensity and frequency of heat waves. This means stronger events in regions already known for heat waves, such as the midwestern and southern U.S. and the Mediterranean region, but it could also mean new risks in regions that at present experience relatively mild heating events, such as the northwestern U.S., France, Germany, and the Balkans.

city living
Urbanization poses an additional challenge in regions characterized by heat waves. In a global warming world, health risks associated with heat events come not only from the events themselves but also from changing demographics and land use. Right now, about 3 billion people, half of us, live in cities. Three-fifths of the (human) population is expected to be living in urban areas by 2030, most of them with relatively meager resources.

Cities (and suburbs) are often significantly warmer than surrounding rural areas, by anywhere from 2 to 10 degrees F. This "heat island" effect is a result of changes to surface materials (infrastructure replacing vegetation), changes to near-surface air circulation (stagnant air in the narrow "canyons" between tall buildings), and "waste" heat produced by vehicles and buildings (manufacturing, air conditioners, etc.). Re-radiation of energy absorbed during the day keeps cities warm at night.

Removal of vegetation in favor of paved and built surfaces is an important part of the heat island effect. First, it can increase the amount of solar energy the absorbed by the landscape. Warmer materials re-radiate more heat to near-surface air, warming the air. Second, it reduces evaporation from soil and leaves (evapotranspiration). The phase change from liquid to vapor uses energy and thus has a cooling effect so when evapotranspiration is reduced, so is the cooling effect it provides. A large deciduous tree can evapotranspire up to 40 gallons of water a day, equivalent to several degrees of cooling (U.S. EPA). Third, loss of shading allows surfaces to absorb more incoming solar radiation than would otherwise be the case.

What can be done to cool down the urban heat island? Energy efficiency is a great place to start. Anything that reduces waste heat (like using a more efficient air conditioner or driving less), has an immediate cooling effect.

Replanting the cityscape, both at street level and up on the rooftops, is another great strategy. If rooftop gardens are not a possibility, changing roof color (from dark to light, in order to reflect more incoming radiation) can help, but green roofs have collateral benefits that make them a better choice. An easy-to-read NASA report on urban heat and green rooftops is here.

Cool pavements are another urban heat mitigation option. The basic idea is to change the surface reflectivity and permeability. The former works by reflecting incoming solar radiation and the later by enhancing evaporative cooling. Research into cool paving strategies is relatively new.

Urban health risks are myriad and it may seem daunting to try to overcome them. However, as is the case with many environmental issues, steps taken to cool the urban heat engine will have many collateral benefits. For example, trees and other vegetation help cool cities and they also clean the air, divert storm water from sewer systems, provide wildlife habitat, and sequester carbon. The Sacramento (CA) Municipal Utility District developed a Tree Benefit Estimator available here. If you live in an urban area, there is a good chance that your municipal government (or utility district) has programs and incentives already in place to help you do your part in meeting the challenge. Here's a short list at EPA.